Hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent drawability

ABSTRACT

A hot-rolled steel sheet having a sheet thickness of 2-10 mm and containing specific amounts of C, Mn, Al, and N with iron and unavoidable impurities. With regard to all grains existing at a position of t/4 in depth, t being sheet thickness, an area ratio of grains having a sheet-plane orientation within 10° from (123) plane is 20% or more, a total area ratio of grains having a crystal direction within 10° from &lt;001&gt; direction and grains having a crystal direction within 10° from &lt;110&gt; direction, in a rolling direction, is 25% or less, and an average grain size of all grains is 3-50 μm.

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet exhibiting good cold formability during processing before a heat treatment and exhibiting a predetermined surface hardness and a desired hardness even in a deep portion from the surface after a carburizing heat treatment. More specifically, it relates to, among steel materials used as various structural parts, a hot-rolled steel sheet useful as a material for the manufacture of parts, for example, clutches, dampers, gears, and the like employed in each portion of automobiles, etc, which are subjected to a surface hardening treatment by a carburizing-quenching or carbonitriding-quenching treatment so as to improve the wear resistance and anti-fatigue properties. In the following description, explanations are given by referring typically to a case of application in clutches but the present invention is of course not limited to the manufacture thereof and can be effectively utilized as a material for the manufacture of parts requiring high surface hardness and excellent impact properties by taking advantage of excellent carburizing-quenching performance and carbonitriding-quenching performance thereof to harden the surface portion while maintaining high toughness in the core portion.

BACKGROUND ART

In recent years, from the standpoint of environmental protection, a requirement for weight reduction, i.e., higher strength, of steel materials for use in various parts for automotive, for example, transmission parts such as gears, and casings is more and more increasing with the purpose of enhancing the fuel efficiency of automobiles. To meet this requirement for weight reduction and higher strength, a steel material prepared by hot-forging a steel bar (hot-forged material) has been used as a commonly-employed steel material (for example, see Patent Document 1). In addition, in order to reduce CO₂ emission amount in the process of producing parts, a requirement for cold forging of parts such as gears, which had been heretofore worked by hot forging, is also increasing.

Cold working (cold forging) is advantageous in that the productivity thereof is high compared with hot working and warm working and moreover, both the dimensional accuracy and the steel material yield are good. On the other hand, a problem occurring in the case of producing parts by the cold working is that a steel material having high strength, i.e., high deformation resistance, must be necessarily used so as to ensure that the strength of cold-worked parts is equal to or more than a predetermined value expected. However, a steel material with a higher deformation resistance to be used has a disadvantage of leading to a shortening of the life of a metal mold for cold working.

Under the above-mentioned background, in the field of transmission parts, studies has been carried out to switch from the conventional forged products (e.g., hot-forged and cold-forged) of steel bars to the manufacture of parts using steel sheets, with an aim of achieving weight reduction or cost reduction of parts. Among others, in the parts of which surface is exposed to a contact pressure, such as gears, dampers and clutches, the surface hardness is increased by applying a carburizing heat treatment after machining of a steel sheet into parts, so as to impart wear resistance and anti-fatigue properties. As the steel sheet for the manufacture of these parts, a general soft steel (e.g., SPHC) has been conventionally used, but further higher strength and higher hardness are demanded.

High-strength parts assured of predetermined strength and surface hardness are manufactured by performing a carburizing heat treatment after cold forming (e.g., press forming) of a steel sheet into a predetermined shape. In order to increase the hardness of the carburized surface, it may be thought to increase the amount of a principal component, mainly the C amount, or of an additive element, but this causes a reduction in the cold formability before the heat treatment. Accordingly, a solution capable of achieving both of ensuring the cold formability and enhancing the surface hardness after a carburizing heat treatment has been required.

As described above, the present invention targets a hot-rolled steel sheet. Conventional techniques related to a hot-rolled steel sheet include, for example, the following Patent Documents 2 to 6.

The hot-rolled steel sheet disclosed in Patent Document 2 is supposed to be enhanced in formability by virtue of a configuration where the average r value is 1.2 or more, the r value (rL) in the rolling direction is 1.3 or more, the r value (rD) in the 45° direction with respect to the rolling direction is 0.9 or more, the r value (rC) in a direction perpendicular to the rolling direction is 1.2 or more, and the X-ray reflection surface random intensity ratios of {111}, {100} and {110} in the sheet plane at ½ sheet thickness of the steel sheet are 2.0 or more, 1.0 or more, and 0.2 or more, respectively.

The hot-rolled steel sheet disclosed in Patent Document 3 is supposed to be enhanced in the ductility and burring formability by virtue of a configuration where the steel sheet has a microstructure containing from 40 to 95% by volume of bainite phase, with the remainder being a ferrite phase, the average grain diameter of the ferrite is 1.2 μm or more and less than 4 μm, and at least either one property of the following (a) and (b) is satisfied:

(a) the ratio (ds/dc) between the average grain diameter (ds) of ferrite at a position of ⅛ thickness in the sheet thickness direction from the surface of the steel sheet and the average grain diameter (dc) of ferrite at the center of sheet thickness is from 0.3 to 0.7; and

(b) the sum of pole densities of {110}<111>, {110}<001> and {211}<111> at a position of ⅛ thickness in the sheet thickness direction from the surface of the steel sheet is 5 times or more of one not having a texture, and each pole density is 1.5 times or more.

The hot-rolled steel sheet disclosed in Patent Document 4 is supposed to be enhanced in rigidity by virtue of a configuration where the average grain size of a ferrite phase is 5 μm or less, the steel sheet has a microstructure allowing a ferrite phase to be present at an area ratio of 50% or more, and the average ODF analysis intensity fin (113)[1-10] to (223)[1-10] orientations in the sheet plane at ¼ sheet thickness of the steel sheet is 4 or more.

The hot-rolled steel sheet disclosed in Patent Document 5 is supposed to have enhanced draw formability by virtue of a configuration where Δr1 specified by the following formula (1) is −0.20 or more and 0.20 or less and Δr2 specified by the following formula (2) is 0.42 or less.

Δr1=(r0−2r45+r90)/2  (1)

Δr2=rmax−rmin  (2)

Each symbol in the formulae represents the following value:

r0: an r value measured on a specimen sampled in parallel with the rolling direction of sheet plane,

r45: an r value measured on a specimen sampled in the 45° direction with respect to the rolling direction of sheet plane,

r90: an r value measured on a specimen sampled in the 90° direction with respect to the rolling direction of sheet plane,

rmax: a maximum value among r0, r45 and r90, and

rmin: a minimum value among r0, r45 and r90.

The hot-rolled steel sheet disclosed in Patent Document 6 is supposed to have enhanced stretch flange formability by virtue of a configuration where the ferrite average grain size is from 1 to 10 μm, the standard deviation of the ferrite grain size is 3.0 μm or less and the shape ratio of an inclusion is 2.0 or less.

Although the hot-rolled steel sheets disclosed in Patent Documents 2 to 6 are excellent in the cold formability such as drawability, the surface hardness after a carburizing heat treatment is not referred to at all, and the improvement effect thereon is unknown.

On the other hand, the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 7 is supposed to enable reducing the “shear droop” during stamping and omitting a carburizing treatment after the stamping by virtue of a configuration where the average hardness to a depth of 50 μm in the surface layer part in the sheet thickness direction is 170 HV or more, the metal microstructure is ferrite+pearlite, and the difference ΔC=CS−CM between the surface carbon concentration CS (mass %) and the in-steel average carbon concentration CM (mass %) is 0.1 mass % or more.

Although the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 7 is excellent in the surface hardness after a carburizing heat treatment, cold formability as well as drawability are not referred to at all, and the improvement effect thereon is unknown.

As described above, almost no studies have been heretofore made on a hot-rolled steel sheet having both drawability and surface hardness after a carburizing heat treatment.

PRIOR ART LITERATURE Patent Documents

Patent Document 1: Japanese Patent No. 3,094,856

Patent Document 2: Japanese Patent No. 3,742,559

Patent Document 3: Japanese Patent No. 4,161,935

Patent Document 4: Japanese Patent No. 4,867,257

Patent Document 5: JP-A-2013-119635

Patent Document 6: Japanese Patent No. 4,276,504

Patent Document 7: JP-A-2010-222663

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The object of the present invention is to provide a hot-rolled steel sheet having both drawability and surface hardness after a carburizing heat treatment. In the present invention, the carburizing heat treatment encompasses also the case of a heat treatment for carbonitridation, in addition to for normal carburization.

Means for Solving the Problems

The invention according to claim 1 is a hot-rolled steel sheet excellent in drawability and surface hardness after a carburizing heat treatment, having:

a sheet thickness of from 2 to 10 mm;

a component composition containing, in mass % (hereinafter, the same applies to chemical components),

C: from 0.05 to 0.30%,

Mn: from 0.3 to 3.0%,

Al: from 0.015 to 0.1%, and

N: from 0.003 to 0.30%,

with the remainder being iron and unavoidable impurities; and

a microstructure mainly containing ferrite and pearlite, in which

with respect to all of grains including the ferrite and pearlite (hereinafter, referred to as “all grains”), existing at a position of t/4 in depth (t: sheet thickness),

an area ratio of grains having a sheet-plane orientation within 10° from (123) plane is 20% or more,

a total area ratio of grains having a crystal direction within 10° from <001> direction and grains having a crystal direction within 10° from <110> direction, in a rolling direction, is 25% or less, and

an average grain size of the all grains is from 3 to 50 μm.

The invention according to claim 2 is the hot-rolled steel sheet according to claim 1, in which, in the unavoidable impurities, Si is 0.5% or less, P is 0.030% or less and S is 0.035% or less.

The invention according to claim 3 is the hot-rolled steel sheet according to claim 1 or 2, in which the component composition further contains at least one member of the following (a) to (f):

(a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%);

(b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%);

(c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%);

(d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%);

(e) Sb: 0.02% or less (exclusive of 0%); and

(f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).

ADVANTAGE OF THE INVENTION

According to the present invention, in a microstructure mainly containing ferrite+pearlite, by controlling the texture of a hot-rolled steel sheet to a predetermined microstructure configuration and thereby increasing the deformability during processing even in parts requiring drawability, the life of a metal mold could be extended, and a hot-rolled steel sheet hardly allows generation of cracking in the steel sheet and can ensure a predetermined surface hardness for parts obtained after a carburizing heat treatment can be provided.

MODE FOR CARRYING OUT THE INVENTION

The hot-rolled steel sheet according to the present invention (hereinafter, sometimes referred to as “steel sheet of the present invention” or simply as “steel sheet”) is described in more detail below. The steel sheet of the present invention is common with the hot-forged material (high-strength high-toughness steel for case-hardening) described in Patent Document 1 in terms of the component composition but differs in that the microstructure is a microstructure mainly containing ferrite+pearlite and not only the texture of the hot-rolled steel sheet is controlled to a predetermined microstructure configuration but also the grains are refined.

Sheet Thickness of Steel Sheet of the Present Invention: from 2 to 10 mm

The steel sheet of the present invention targets one having a sheet thickness of 2 to 10 mm. If the sheet thickness is less than 2 mm, the rigidity as a structure cannot be ensured. On the other hand, if the sheet thickness exceeds 10 mm, the microstructure configuration specified in the present invention can be hardly achieved, and the desired effects cannot be obtained. The lower limit of the sheet thickness is preferably 3 mm or more and more preferably 4 mm or more. The upper limit thereof is preferably 9 mm or less and more preferably 7 mm or less.

Next, the component composition constituting the steel sheet of the present invention is described. In the following, all of the units of chemical components are mass %.

Component Composition of Steel Sheet of the Present Invention

<C: from 0.05 to 0.30%>

C is an element indispensable for ensuring the strength of the core portion as a carburizing—(or carbonitriding—) quenched part finally obtained, and if it is less than 0.05%, sufficient strength cannot be obtained. However, if it is excessively contained, not only the toughness is deteriorated but also the machinability or cold forgeability is reduced to impair the formability, and therefore, the upper limit thereof is 0.30%. The preferred content of C is in the range of from 0.08 to 0.25%.

<Mn: from 0.3 to 3.0%>

Mn is an element effective for the deoxidation of molten steel and in order to effectively bring out such an effect, it must be contained in an amount of 0.3% or more. If it is excessively contained, cold formability or machinability is adversely affected and the segregation amount at grain boundary is increased to decrease the grain boundary strength, resulting in an adverse effect on the impact properties. Therefore, the content thereof must be 3.0% or less. The preferred content of Mn is in the range of from 0.5 to 2.0%.

<Al: from 0.015 to 0.1%>

Al is an element contained in the steel as a deoxidizer for the steel material and has an action of binding to N in the steel to form AlN and thereby preventing grain growth.

In order to effectively bring out such an effect, it must be contained in an amount of 0.015% or more. The effect is saturated at about 0.1%, and if the content exceeds it, the element binds with oxygen to form a nonmetallic inclusion and adversely affects impact properties, etc. Therefore, the upper limit thereof has been specified to be 0.1%, and is preferably 0.08% or less, more preferably 0.06% or less and especially preferably 0.04% or less.

<N: from 0.003 to 0.30%>

N has an action of binding to Al, V, Ti, Nb, etc. in the steel to form a nitride and thereby suppressing grain growth, and this effect is effectively exerted when it is contained in an amount of 0.003% or more. It is preferably 0.005% or more. However, such an effect is saturated at about 0.30%, and if contained by not less than the above amount, the nitride works as an inclusion and adversely affects the physical properties. Therefore, the upper limit has been specified to be 0.30%, and is preferably 0.10% or less, more preferably 0.05% or less and especially preferably 0.03% or less.

The steel sheet of the present invention fundamentally contains the above-described components, with the remainder being iron and unavoidable impurities. The contents of Si, P and S unavoidably getting mixed in are desirably kept as small as possible for the following reasons.

<Si: 0.5% or less>

Si effectively acts as a strengthening element or a deoxidizing element but, on the other hand, promotes grain boundary oxidation to deteriorate the bending fatigue properties and adversely affects the cold forgeability. Accordingly, in order to remove such the problems, the content thereof must be kept at 0.5% or less and, among others, when high-level bending fatigue properties are required, the content thereof is preferably kept at 0.1% or less. From such a viewpoint, the more preferred content of Si is in the range of from 0.02 to 0.1%.

<P: 0.030% or less>

P segregates at the grain boundary to reduce the toughness and therefore, the upper limit thereof has been specified to be 0.030%. The more preferred content of P is 0.020% or less and further preferably 0.010% or less.

<S: 0.035% or less>

S produces MnS and contributes to enhancement of machinability. In the case of applying the present invention to gears, etc., not only vertical impact properties but also lateral impact properties are important, and the anisotropy needs to be reduced to enhance the lateral impact properties. For this purpose, the S content must be kept at 0.035% or less. The more preferred content of S is 0.025% or less and further preferably 0.020% or less.

The steel sheet of the present invention may contain the following tolerable components, in addition to the above-described basic components, in the ranges not impairing the actions of the present invention.

<At least one member selected from the group consisting of

Cr: 3.0% or less (exclusive of 0%),

Mo: 1.0% or less (exclusive of 0%) and

Ni: 3.0% or less (exclusive of 0%)>

These elements are useful elements in terms of having an action of improving the quenchability or refining the quenched microstructure. In particular, Cr has an excellent effect of enhancing the quenchability; Mo effectively acts to decrease an incompletely quenched microstructure, enhance the quenchability and furthermore, increase the grain boundary strength; and Ni refines the microstructure after quenching and thereby contributes to enhancement of impact resistance. These effects are effectively exerted by preferably containing at least one member of Cr: 0.2% or more, Mo: 0.08% or more and Ni: 0.2% or more. However, if the Cr amount exceeds 3.0%, Cr produces a carbide and causes grain boundary segregation to reduce the grain boundary strength and in turn, adversely affect the toughness; the above-described effects of Mo are saturated at about 1.0%; and the above-described effects of Ni are also saturated at 3.0%. Therefore, addition by not less than those amounts is utterly useless from an economical viewpoint.

<Cu: 2.0% or less (exclusive of 0%) and/or

Co: 5% or less (exclusive of 0%)>

Cu is an element effectively acting to enhance the corrosion resistance, and the effect is effectively exerted by being contained in an amount of preferably 0.3% or more. However, the effect is saturated at a content of 2.0% and therefore, containing by not less than the above amount is useless. When Cu is contained alone, the hot formability of the steel material tends to be deteriorated and in order to avoid such an ill effect, Ni having an effect of enhancing the hot formability is desirably used in combination in the above-described content range.

In addition, both Cu and Co are elements having an action of causing strain-aging and hardening of a steel material and effective for enhancing the strength after processing. In order to effectively bring out such the actions, these elements are preferably contained each in an amount of 0.1% or more and furthermore 0.3% or more. However, if the Co content is excessively large, the effect of causing strain-aging and hardening of a steel material and the effect of enhancing the strength after processing may be saturated, or cracking may be encouraged. Therefore, it is recommended that the Co content is 5% or less, furthermore 4% or less and particularly 3% or less.

<At least one member selected from the group consisting of

V: 0.5% or less (exclusive of 0%),

Ti: 0.1% or less (exclusive of 0%) and

Nb: 0.1% or less (exclusive of 0%)>

These elements contribute to enhancing the toughness (impact resistance) by binding to C or N to produce a carbide or a nitride and by thus refining the grains. However, since the effect is saturated around each upper limit value and the machinability or cold formability may be rather adversely affected, they must be kept equal to or less than the respective upper limit values. The preferable lower limit values for effectively bringing out the addition effect of these elements are V: 0.03%, Ti: 0.005% and Nb: 0.005%.

<Ca: 0.08% or less (exclusive of 0%) and/or

Zr: 0.08% or less (exclusive of 0%)>

Ca envelops a hard inclusion in a flexible inclusion, and Zr spheroidizes MnS, both thereby contributing to enhancing the machinability. In addition, both elements have an effect of increasing the lateral impact properties by virtue of the reduction in the anisotropy by the spheroidization of MnS. However, these effects are each saturated at 0.08% and therefore, it is recommended that the content of each is 0.08% or less, furthermore 0.05% or less and particularly 0.01% or less. The preferable lower limit values for effectively bringing out the above-described effects of these elements are Ca: 0.0005% (furthermore 0.001%) and Zr: 0.002%.

<Sb: 0.02% or less (exclusive of 0%)>

Sb is an effective element for suppressing the grain boundary oxidation and thereby increasing the bending fatigue strength. However, since the effect is saturated at 0.02%, addition by not less than the amount is useless from an economical viewpoint. The preferable lower limit value for effectively bringing out the addition effect of Sb is 0.001%.

<At least one member selected from the group consisting of

REM: 0.05% or less (exclusive of 0%),

Mg: 0.02% or less (exclusive of 0%),

Li: 0.02% or less (exclusive of 0%),

Pb: 0.5% or less (exclusive of 0%), and

Bi: 0.5% or less (exclusive of 0%)>

REM is, similarly to Zr and Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and contributing to enhancement of the machinability. In order to effectively bring out such actions, REM is preferably contained in an amount of 0.0005% or more and furthermore 0.001% or more. However, even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, recommended are 0.05% or less, furthermore 0.03% or less and particularly 0.01% or less.

The “REM” in the present invention means to include lanthanoid elements (15 elements from La to Ln) as well as Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, and it is more preferable to contain La and/or Ce.

Mg is, similarly to Zr and Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and contributing to enhancement of the machinability. In order to effectively bring out such actions, Mg is preferably contained in an amount of 0.0002% or more and furthermore 0.0005% or more. However, even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, recommended are 0.02% or less, furthermore 0.015% or less and particularly 0.01% or less.

Li is, similarly to Zr and Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to allow for enhancement of the deformation performance of steel and contributing to improvement of the machinability by lowering the melting point of an Al-based oxide and thereby making it harmless. In order to effectively bring out such actions, Li is preferably contained in an amount of 0.0002% or more and furthermore 0.0005% or more. However, even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, recommended are 0.02% or less, furthermore 0.015% or less and particularly 0.01% or less.

Pb is an element effective for enhancing the machinability. In order to effectively bring out such an action, Pb is preferably contained in an amount of 0.005% or more and furthermore 0.01% or more. However, if contained too much, there arises a problem with production such as formation of a roll mark. Therefore, recommended are 0.5% or less, furthermore 0.4% or less and particularly 0.3% or less.

Bi is, similarly to Pb, an element effective for enhancing the machinability. In order to effectively bring out such an action, Bi is preferably contained in an amount of 0.005% or more and furthermore 0.01% or more. However, even if contained too much, the effect of enhancing the machinability is saturated. Therefore, recommended are 0.5% or less, furthermore 0.4% or less and particularly 0.3% or less.

The microstructure characterizing the steel sheet of the present invention is described below.

Microstructure of Steel Sheet of the Present Invention

As described above, the steel sheet of the present invention has a microstructure mainly containing ferrite and pearlite and in particular, is characterized in that the texture configuration in steel is more strictly controlled. In the present invention, the microstructure mainly containing ferrite and pearlite means that the total amount of ferrite and pearlite is 90% or more in terms of the area ratio. As long as the total amount of ferrite and pearlite is 90% or more in terms of the area ratio, other microstructures (e.g., bainite, martensite) may be produced in a small amount, but the amount of other microstructures is preferably as small as possible.

In general, with respect to the texture control for the enhancement of formability of a steel sheet, both experiments and theory have heretofore clarified that, as for the deep drawability of a thin steel sheet for use in a body outer panel of an automobile, as the plastic anisotropy (r value (Lankford value): the ratio of a sheet-width strain to a sheet-thickness strain in a tensile test) of a material is larger, the drawability is higher and furthermore, it is essential for the enhancement of deep drawability to strongly develop the {111} plane parallel with a sheet-plane orientation and weaken the {100} plane orientation in the recrystallization texture (see, Iron and Steel Institute of Japan: “Recrystallization, Texture and Their Application to Structural Control”, March, 1999, p. 208).

Therefore, in a steel sheet for the body outer panel, various approaches to enhancing the formability by the texture control are taken as disclosed in Patent Documents 2 to 4, but such approaches have not been attempted in a steel sheet for a carburizing heat treatment.

Texture of Steel Sheet of the Present Invention

The steel sheet of the present invention is characterized in that, with respect to grains of all phases including ferrite and pearlite, the orientation and size of the grains are controlled to specific ranges.

With respect to all of grains including ferrite and pearlite (hereinafter, referred to as “all grains”), existing at a position of t/4 in depth (t: sheet thickness),

an area ratio of grains having a sheet-plane orientation within 10° from (123) plane is 20% or more>

How the texture is formed differs depending on the processing method, despite the same crystal system, and in the case of a rolled material, it is expressed by a rolling surface and a rolling direction. As described below, the rolling surface is represented by {ooo}, and the rolling direction is represented by <ΔΔΔ>. Here, o or Δ indicates an integer. The expression of each orientation is described, for example, in Shinichi Nagashima (compiler), “Texture” (published by Maruzen Co. Ltd.).

In the present invention, with respect to all grains existing at a position of t/4 in depth, the area ratio of those having a sheet-plane orientation within 10° from (123) plane is controlled to be 20% or more, whereby the cold formability and drawability of a steel sheet for a carburizing heat treatment can be enhanced.

With respect to the sheet-plane orientation of grains, it has been conventionally known to be effective in enhancing the deep drawability to strongly develop the (111) plane orientation parallel with the sheet plane and on the other hand, weaken the (001) plane orientation. Such a control of the sheet-plane orientation may have been possible in the process involving applying a cold rolling step and an annealing step, but this sheet-plane orientation control has been difficult in the steel sheet of the present invention.

Therefore, in the present invention, a grain having a sheet-plane orientation of (123) plane is newly introduced, whereby a texture control can be achieved in the steel sheet of the present invention and enhancement of the cold formability in a softened state can be realized.

As described above, a grain having (123) plane as the sheet-plane orientation has an action of enhancing the cold formability in a softened state, and in order to effectively bring out such an action, 20% or more in terms of area ratio is needed. It is preferably 22% or more, more preferably 24% or more and still more preferably 26% or more.

Here, since a sheet material has a microstructure distribution in the sheet thickness direction, the microstructure configuration has been specified by employing, as the representative position, a position of ¼ sheet thickness in depth. In addition, since grains having a sheet-plane orientation within 10° from the above-described ideal plane orientation ((123) plane) are considered to have a substantially equivalent action, the microstructure is specified by the area ratio of grains having a sheet-plane orientation in this range.

<A total area ratio of grains having a crystal direction within 10° from <001> direction and grains having a crystal direction within 10° from <110> direction, in a rolling direction, is 25% or less>

As this total area ratio is larger, the in-plane anisotropy in drawability increases, and for this reason, the ratio is limited to 25% or less, preferably 23% or less and more preferably 20% or less.

In addition, due to the above-described effect of reducing the in-plane anisotropy, martensite transformation proceeds in quenching after a carburizing heat treatment while maintaining the original crystal orientation relationship of grains, as a result, an effect of reducing transformation strain and enhancing the dimensional accuracy of parts can also be obtained. In another interpretation, it may also be thought that occurrence of in-plane anisotropy during the forming process indicates generation of unevenness in the strain in the parts and since this causes deterioration of the dimensional accuracy of parts after carburizing heat treatment and quenching, the effect of reducing the in-plane anisotropy by the above-described texture control brings about an effect of enhancing the dimensional accuracy of parts.

<An average grain size of the all grains is from 3 to 50 μm>

The average grain size of all grains needs to be in the range of from 3 to 50 μm so as to enhance the formability (drawability, bendability, press formability) of steel sheet and to satisfy the surface quality after processing. If the grains are excessively refined, the deformation resistance is too much increased, and therefore, the average grain size is set to be 3 μm or more, preferably 4 μm or more, and more preferably 5 μm or more. On the other hand, if the grains grow excessively, not only the toughness, fatigue properties, etc. are deteriorated but also the bendability or press moldability such as overhanging is significantly reduced even when the crystal orientation is controlled, and cracking during forming process or a defect such as surface roughening is likely to be generated. Therefore, the average grain size is set to be 50 μm or less, preferably 45 μm or less and more preferably 40 μm or less. Similarly to the above, since a size distribution of grains is present in the sheet thickness direction, the average grain size of all grains has been specified by employing, as the representative position, a position of ¼ sheet thickness in depth.

Method for Measuring Sheet-Plane Orientation of Grain

The sheet-plane orientation of the grain is measured/analyzed by means of SEM-EB SP (Electron Back Scattering Pattern) and EBSD (Electron Back Scattering Diffraction). For example, SEM (JEOLJSM5410) manufactured by JEOL Ltd. is used as the SEM apparatus and, for example, EB SP (OM) manufactured by TSL Corp. is used as the EBSP measurement/analysis system. Although these may vary depending on the size of the grain, the sample measurement region is set to 300 to 1,000 μm×300 to 1,000 μm and the measurement step interval is set, for example, to 1 to 3 μm. Out of the crystal orientations of respective grains thus identified, those having an orientation within 10° from each ideal plane orientation are totalized to determine a total area, followed by dividing by the area of the measurement region, whereby an area ratio is determined for every ideal plane orientation.

Method for Measuring Crystal Direction in Rolling Direction of Grain

As for the crystal direction in the rolling direction of the grain, the cross-section (side surface) in the rolling direction of the steel sheet is measured with EBSP, and grains having a crystal direction within 10° from <001> direction and grains having a crystal direction within 10° from <110> direction, in the rolling direction, are identified by analysis. With respect to the measuring method, although these may vary depending on the size of the grain, the sample measurement region is set to 300 to 1,000 μm×300 to 1,000 μm at a ¼ part in the sheet thickness direction and the measurement step interval is set, for example, to 1 to 3 μm. Out of the crystal directions of respective grains thus identified, those having an orientation within 10° from each ideal plane direction are totalized to determine a total area, followed by dividing by the area of the measurement region, whereby an area ratio is determined for every ideal crystal direction.

Method for Measuring Average Grain Size of the All Grains

As for the average grain size of the above-described all grains, maximum diameters of each grain observed in a predetermined measurement region were measured by employing SEM-EBSP above and the measurement conditions thereof, and an average value of measured values was determined as the average grain size.

The preferable production method for obtaining the steel sheet of the present invention is described below.

Preferable Production Method of Steel Sheet of the Present Invention

The steel sheet of the present invention can be produced, for example, as a hot-rolled coil obtained by melting a raw material steel having the above-described component composition, casting it to form a slab, and subjecting the slab as it is or the slab after surface chamfering to respective steps of heating, hot rough rolling and finish rolling. Pickling and skin pass rolling may be thereafter further applied according to the required conditions such as surface state and sheet thickness accuracy.

Preparation of Molten Steel

First, desired oxides can be produced by adding predetermined alloy elements in a predetermined order to a molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted. Above all, in the present invention, it is very important to adjust the dissolved oxygen amount and thereafter, adjust the total oxygen amount so as to inhibit production of a coarse oxide.

The “dissolved oxygen” means oxygen that is present in the molten steel without forming an oxide and kept in a free state. The “total oxygen” means the total of all oxygens contained in the molten steel, i.e., free oxygen and oxygen forming an oxide.

The dissolved oxygen amount in the molten steel is first adjusted to a range of 0.0010 to 0.0060%. If the dissolved oxygen amount in the molten steel is less than 0.0010%, a predetermined amount of Al—O-based oxide cannot be ensured due to shortage of the dissolved oxygen amount in the molten steel, and a desired size distribution cannot be obtained. In addition, if the dissolved oxygen amount is insufficient, in the case of adding REM, the REM forms a sulfide, and an inclusion is thereby coarsened, giving rise to deterioration of the properties. Therefore, the dissolved oxygen amount is set to be 0.0010% or more. The dissolved oxygen amount is preferably 0.0013% or more and more preferably 0.0020% or more.

On the other hand, if the dissolved oxygen amount exceeds 0.0060%, not only the reaction of oxygen and the elements above in the molten steel becomes vigorous due to an excessively large oxygen amount in the molten steel, which is disadvantageous in view of melting operation, but also a coarse oxide is produced to rather deteriorate the properties. Therefore, the dissolved oxygen amount should be kept at 0.0060% or less. The dissolved oxygen amount is preferably 0.0055% or less and more preferably 0.0053% or less.

The dissolved oxygen amount in a molten steel having been subjected to primary refining in a converter or an electric furnace usually exceeds 0.010%. Therefore, in the production method of the present invention, the dissolved oxygen amount in the molten steel needs to be adjusted to the range above in some way.

The method for adjusting the dissolved oxygen amount in the molten steel includes, for example, a method of performing vacuum C deoxidation by using an RH-type degassing refining apparatus and a method of adding a deoxidizing element such as Si, Mn and Al, and the dissolved oxygen amount may also be adjusted by appropriately combining these methods. In addition, the dissolved oxygen amount may be adjusted by using a ladle heating-type refining apparatus, a simple molten metal treatment system, etc., in place of the RH-type degassing refining apparatus. In this case, since the dissolved oxygen amount cannot be adjusted by vacuum C deoxidation, a method of adding a deoxidizing element such as Si may be employed for the adjustment of the dissolved oxygen amount. In the case of employing the method of adding a deoxidizing element such as Si, the deoxidizing element may be added when the steel is tapped from the converter to the ladle.

After adjusting the dissolved oxygen amount in the molten steel to the range of 0.0010 to 0.0060%, the molten steel is stirred to float and separate an oxide in the molten steel, whereby the total oxygen amount in the molten steel is adjusted to the range of 0.0010 to 0.0070%. Thus, in the present invention, after removing unnecessary oxides by stirring a molten steel in which the molten oxygen amount is appropriately controlled, the production of a coarse oxide, i.e., a coarse inclusion, can be prevented.

If the total oxygen amount is less than 0.0010%, the desired amount of oxide lacks and therefore, the amount of oxide contributing to a fine size distribution of inclusions cannot be ensured. Therefore, the total oxygen amount is set to be 0.0010% or more. The total oxygen amount is preferably 0.0015% or more and more preferably 0.0018% or more.

On the other hand, if the total oxygen amount exceeds 0.0070%, the amount of oxide in the molten steel is excessively large, as a result, a coarse oxide, i.e., a coarse inclusion, is produced to deteriorate the properties. Therefore, the total oxygen amount should be kept at 0.0070% or less. The total oxygen amount is preferably 0.0060% or less and more preferably 0.0050% or less.

The total oxygen amount in the molten steel generally varies in a correlated manner in response to the stirring time of the molten steel and therefore, can be controlled, for example, by adjusting the stirring time. Specifically, the total oxygen amount in the molten steel is appropriately controlled while stirring the molten steel and measuring from time to time the total oxygen amount in the molten steel after removing an oxide floated.

In the case of adding REM and Ca to the steel material, after adjusting the total oxygen amount in the molten steel to the above-described range, REM is added and casting is then performed. A desired oxide can be obtained by adding the elements above to a molten steel in which the total oxygen amount has been adjusted.

The forms of REM and Ca to be added to the molten steel are not particularly limited and, for example, pure La, pure Ce, pure Y, etc. as REM, or pure Ca, and furthermore, Fe—Si—La alloy, Fe—Si—Ce alloy, Fe—Si—Ca alloy, Fe—Si—La—Ce alloy, Fe—Ca alloy, or Ni—Ca alloy may be added. A misch metal may also be added to the molten metal. The misch metal is a mixture of cerium group rare-earth elements and, specifically, contains approximately from 40 to 50% of Ce and approximately from 20 to 40% of La. However, the misch metal often contains Ca as an impurity and in the case where the misch metal contains Ca, the preferable range specified in the present invention must be satisfied.

In the case where REM is added, in the present invention, the molten steel after the addition of REM is preferably stirred for in the range of not more than 40 minutes so as to promote the removal of a coarse oxide. If the stirring time exceeds 40 minutes, an oxide is coarsened due to occurrence of aggregation/coalescence of fine oxides in the molten steel, whereby the properties are deteriorated. Therefore, the stirring time is preferably 40 minutes or less. The stirring time is more preferably 35 minutes or less and still more preferably 30 minutes or less. The lower limit value of the stirring time of the molten steel is not particularly limited, but if the stirring time is too short, the concentrations of additive elements are non-uniform, and the desired effect as the entire steel material cannot be obtained. Accordingly, a desired stirring time in accordance with the container size is required.

In this way, a molten steel having an adjusted component composition can be obtained. By using the obtained molten steel, casting is performed to obtain a billet.

Next, heating, hot rolling including finish rolling, rapid cooling after hot rolling, slow cooling after stop of rapid cooling, rapid cooling and coiling after slow cooling are performed for the production.

Heating

The heating before hot rolling is performed at 1,150 to 1,300° C. This heating provides for an austenite single phase, whereby a solid solution element (including an additive element such as V and Nb) is dissolved in solid in the austenite. If the heating temperature is less than 1,150° C., the element cannot be dissolved in solid in the austenite, and a coarse carbide is formed, as a result, an effect of improving the fatigue properties cannot be obtained. On the other hand, a temperature exceeding 1,300° C. is difficult in view of operation. In the case of containing Ti as an additive element, from the standpoint of forming a solid solution of Ti, which has a highest solution treatment temperature among carbides, a temperature equal to or more than the solution treatment temperature of TiC and 1,300° C. or less is necessary. The more preferred lower limit of the heating temperature is 1,200° C.

Hot Rough Rolling

In the rough rolling, the microstructure control of recrystallized austenite is performed so as to ensure the percentage of a grain with a predetermined crystal orientation specified in the present invention. Taking into account to ensure the temperature in the subsequent finish rolling, the rough rolling temperature is set to be from 900 to 1,200° C., and the austenite grain is refined and repeatedly recrystallized in the rough rolling, whereby the percentage of the grain with a predetermined crystal orientation can be controlled. The rough rolling temperature is more preferably from 900 to 1,100° C.

Hot Finish Rolling

Hot rolling is performed so that the finish rolling temperature can be 800° C. or more. If the finish rolling temperature is set too low, ferrite transformation occurs at a high temperature, and a precipitated carbide in ferrite is coarsened. Therefore, the finish rolling temperature needs to be not less than a given level. The finish rolling temperature is more preferably set to be 850° C. or more so that the austenite grain can grow and the grain size of bainite can be increased.

Rolling Reduction in Final Pass of Hot Finish Rolling

If the rolling reduction in the final pass of the hot finish rolling is too high, the texture configuration of the present invention cannot be obtained, and the anisotropy increases. On the contrary, if the rolling reduction is too low, the texture cannot be developed. Therefore, the rolling reduction in the final pass of hot finish rolling is set to be from 10 to 18%, preferably from 11 to 17% and particularly preferably from 12 to 16%.

Rapid Cooling After Hot Rolling

Within 5 seconds after the completion of the finish rolling, rapid cooling is performed at a cooling rate (rapid cooling rate) of 20° C./s or more, and the rapid cooling is stopped at a temperature (rapid cooling stop temperature) of 580° C. or more and less than 680° C. This is done for lowering the ferrite transformation start temperature and thereby refining the precipitated carbide formed in ferrite. If the cooling rate (rapid cooling rate) is less than 20° C./s, pearlite transformation is promoted, or if the rapid cooling stop temperature is less than 580° C., pearlite transformation or bainite transformation is promoted and, as a result, cold formability is reduced. On the other hand, if the rapid cooling stop temperature is 680° C. or more, the precipitated carbide in ferrite is coarsened, and the anti-fatigue properties cannot be ensured. The rapid cooling stop temperature is preferably from 600 to 650° C. and more preferably 610 to 640° C.

Slow Cooling After Stop of Rapid Cooling

After the stop of the rapid cooling, slow cooling is performed at a cooling rate (slow cooling rate) of 5° C./s or more and less than 20° C./s. The slow cooling rate is set to be 5° C./s or more so as to suppress formation of proeutectoid ferrite during hot rolling, appropriately refine the precipitated carbide in ferrite, and control the grain microstructure in the hot-rolled sheet, thereby controlling the texture configuration in the final steel sheet. If the slow cooling rate is less than 5° C./s, not only the amount of proeutectoid ferrite formed is increased, allowing production of a coarse grain, but also a coarse grain is formed in the final steel sheet to cause a non-uniform state of carbide and deteriorate the cold formability. If the cooling rate is 20° C./s or more, hard phases (bainite and martensite) are more produced, and the cold formability is thereby reduced.

Rapid Cooling and Coiling After Slow Cooling

After the slow cooling, coiling is performed at more than 550° C. and 650° C. or less. If the coiling temperature exceeds 650° C., many surface oxide scales are formed to deteriorate the surface quality, and on the other hand, if it is less than 550° C., many martensites are formed to reduce the cold formability.

The present invention is described in greater detail below by referring to Examples, but the following Examples are not limiting in nature on the present invention, and the present invention can be implemented by making appropriate changes within the scope conformable to the gist described hereinbefore and hereinafter, and all of them are included in the technical scope of the present invention.

EXAMPLES

A steel having the component composition shown in Table 1 below was melted by a vacuum melting method and cast into an ingot having a thickness of 120 mm, followed by performing hot rolling under the conditions shown in Table 2 below to produce a hot-rolled steel sheet. In all tests, the cooling after the stop of rapid cooling was slow cooling under the conditions of a cooling rate of 10° C./s or less for 5 to 20 seconds.

A test steel containing the chemical components shown in Table 1 was melted by using a vacuum melting furnace (capacity: 150 kg) and cast into 150 kg of an ingot, followed by cooling. When the test steel was melted in the vacuum melting furnace, component adjustment was applied to the elements except for Al, REM and Ca, and the dissolved oxygen amount in the molten steel was adjusted by deoxidation by using at least one element selected from C, Si and Mn. The molten steel in which the dissolved oxygen amount is adjusted was stirred for approximately from 1 to 10 minutes to float and separate oxides in the molten steel, and the total oxygen amount in the molten steel was thereby adjusted.

In the case of adding REM and Ca, they were added to a molten steel in which the total oxygen amount had been adjusted, thereby obtaining a molten steel adjusted for component thereof. Here, REM was added in the form of a misch metal containing about 25% of La and about 50% Ce, and Ca was added in the form of an Ni—Ca alloy, a Ca—Si alloy, or an Fe—Ca green compact.

The obtained ingot was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled sheet having a predetermined thickness. In Table 2, the cooling rate after stop of rapid cooling is not shown, but in each production example, a condition of 10° C./s is employed.

With respect to each of the thus-obtained as-hot rolled sheets, the sheet-plane orientation of grain, the crystal direction in the rolling direction of grain, and the average grain size of all grains were examined by the measuring methods described in the item of “MODE FOR CARRYING OUT THE INVENTION” above. Here, it has been confirmed that in all of Steel Nos. 1 to 27 shown in Table 3, the total amount of ferrite and pearlite was 90% or more in terms of area ratio (the microstructure mainly contained ferrite and pearlite).

In addition, in order to evaluate the drawability on each of the as-hot rolled sheets above, a JIS No. 5 piece was sampled to be angled 0° (parallel with rolling direction), 45° or 90° (perpendicular to rolling direction) with respect to the rolling direction and subjected to a tensile test, and the r value (r0, r45, r90) at each angle was determined. The average r value and the Δr value were calculated according to the following formula. Here, the Δr value is an indicator for evaluating the in-plane anisotropy of the r value.

Average r value=(r0+2×r45+r90)/4

Δr value=(r0+r90)/2−r45

Those where all of r0, r45, r90, and the average r value are 0.85 or more and the Δr value is within ±0.1 were judged as passed.

Furthermore, each of the as-hot rolled sheets above was subjected to a carburizing-quenching test under the following conditions for evaluating the surface hardness after a carburizing heat treatment.

Carburizing-Quenching Conditions

A carburizing treatment was applied by holding at 900° C. for 2.5 hours and further at 850° C. for 0.5 hours in a gas atmosphere with a carbon potential (CP value)=0.8% and thereafter, performed were oil-quenching at 100° C., holding at 160° C. for 2 hours for subjecting to a tempering treatment, and air-cooling.

Surface Hardness After Carburizing Heat Treatment

The Vickers hardness (Hv) was measured by using a Vickers hardness tester under the conditions of load: 1,000 g, measurement position: a position of 0.8 mm in depth from the steel sheet surface, and number of measurements: 5 times, and those where the hardness was 350 Hv or more were judged as passed. Here, the measurement position was set to a position of 0.8 mm in depth from the surface because it was specified as a necessary condition to exhibit desired hardness (strength) even at a deep position from the surface after a carburizing heat treatment.

These measurement results are shown in Table 3 below.

TABLE 1 Steel Spe- Components (mass %) [remainder: Fe and unavoidable impurities] cies C Si Mn P S Al N Cr Mo Ni Cu Co V Ti Nb Ca Zr Sb Others a 0.23 0.05 2.50 0.008 0.012 0.032 0.010 — — — — — — — — — — — — b 0.08 0.10 2.10 0.010 0.018 0.026 0.005 — — — — — — — — — — — — c 0.15 0.06 1.88 0.008 0.012 0.027 0.090 1.03 — — — — — — — — — — — d 0.13 0.02 2.20 0.012 0.020 0.016 0.008 0.99 — — — — — — — — — — — e 0.17 0.12 2.01 0.010 0.021 0.022 0.009 — 0.44 — — — — — — — — — — f 0.21 0.41 1.32 0.010 0.008 0.033 0.010 — — 0.99 — — — — — — — — — g 0.17 0.05 1.40 0.008 0.012 0.032 0.010 1.07 0.01 0.02 0.01 — — — — — — — — h 0.19 0.07 1.52 0.010 0.027 0.022 0.080 1.01 — 0.28 — 0.32 — — — — — — — i 0.18 0.06 1.39 0.009 0.010 0.03  0.090 — — — — — 0.33 — — — — — — j 0.19 0.06 1.41 0.010 0.011 0.031 0.070 — — — — — — 0.05 — — — — — k 0.17 0.07 1.40 0.008 0.012 0.03  0.090 — — — — — — — 0.03 — — — — l 0.18 0.06 1.38 0.007 0.010 0.029 0.010 1.03 0.02 0.02 0.01 — — — — 0.002 — — — m 0.18 0.05 1.39 0.007 0.010 0.029 0.010 0.99 0.02 0.03 — 0.01 — — — — 0.002 — — n 0.17 0.05 1.41 0.007 0.009 0.032 0.080 1.07 0.01 0.02 0.01 — — — — — — 0.008 — o 0.17 0.05 1.41 0.007 0.009 0.032 0.080 1.07 0.01 0.02 0.01 — — — — — — — REM: 0.003, Li: 0.001 p 0.19 0.06 1.36 0.007 0.009 0.029 0.070 1.01 0.01 0.02 0.01 — — — — — — — Mg: 0.001, Pb: 0.001, Bi: 0.05 q 0.03 0.05 2.20 0.008 0.001 0.03  0.009 — — — — — — — — — — — — r 0.31 0.05 2.20 0.007 0.001 0.03  0.009 — — — — — — — — — — — — s 0.17 0.07 0.20 0.007 0.001 0.03  0.009 — — — — — — — — — — — — t 0.17 0.07 3.15 0.007 0.001 0.03  0.009 — — — — — — — — — — — — u 0.17 0.07 2.10 0.007 0.001 0.009 0.009 — — — — — — — — — — — — v 0.17 0.07 2.10 0.007 0.001 0.11  0.009 — — — — — — — — — — — — w 0.17 0.07 2.10 0.007 0.001 0.025 0.310 — — — — — — — — — — — — (—: not added, underlined: outside the scope of the present invention)

TABLE 2 Hot Rolling Conditions Rolling Reduction Heating Rough Rolling Finish Rolling in Final Pass of Rapid Cooling Coiling Thickness of Production Steel Temperature Temperature Temperature Finish Rolling Stop Temperature Temperature Hot-Rolled No. Species (° C.) (° C.) (° C.) (%) (° C.) (° C.) Sheet (mm)  1 a 1250 1119 879 13 600 569 3  2 a 1250 1054 906 11 609 598 8  3* a  1000*  891*  771* 13  528*  453* 5  4* a 1250 1081 871 10 651 585 12*  5* a 1250 1184 826  6* 625 560 5  6 b 1250 1098 876 14 601 552 5  7 c 1250 1188 872 14 678 575 5  8 d 1250  958 858 10 640 578 5  9 e 1250 1148 888 14 652 598 5 10 f 1250 1111 853 15 630 565 5 11 g 1250 1199 838 16 638 578 5 12 h 1250 1178 855 14 637 574 5 13 i 1250 1020 874 17 639 547 5 14 j 1250  995 862 10 599 554 5 15 k 1250  962 878 15 638 607 5 16 l 1250 1020 890 16 655 561 5 17 m 1250 1160 863 13 623 621 5 18 n 1250  985 899 13 654 592 5 19 o 1250 1032 862 15 614 595 5 20 p 1250 1098 845 16 613 561 5 21 q 1250 1016 842 13 617 571 5 22 r 1250 1164 894 10 598 598 5 23 s 1250 1118 871 12 616 556 5 24 t 1250 1144 842 12 605 577 5 25 u 1250 1017 897 14 617 578 5 26 v 1250 1047 892 16 599 557 5 27 w 1250 1036 846 12 646 609 5 (underlined = outside the scope of the present invention, *= outside the recommended range)

TABLE 3 Surface Grain Hardness after Area Ratio Area Ratio of Carburizing of (123) <001> Direction + Average Drawability Hardness Steel Steel Production Plane <110> Direction Grain Size Average Δr at Depth of No. Species No. (%) (%) (μm) r0 r45 r90 r value value 0.8 mm (Hv) Remarks 1 a  1 27 10 25 0.89 0.90 0.87 0.89 −0.021 387 Steel of Invention 2 a  2 42 11 37 0.89 0.90 0.92 0.90  0.002 358 Steel of Invention 3 a  3* 26 32 17 0.80 0.92 0.81 0.86 −0.115 393 Comp. steel 4 a  4* 15 12 56 0.81 0.85 0.82 0.83 −0.035 382 Comp. steel 5 a  5* 16 29 15 0.81 0.91 0.80 0.86 −0.110 394 Comp. steel 6 b  6 31 16 24 0.87 0.88 0.91 0.89  0.008 398 Steel of Invention 7 c  7 30 20 23 0.88 0.89 0.90 0.89 −0.004 401 Steel of Invention 8 d  8 37 15 17 0.86 0.87 0.91 0.88  0.015 405 Steel of Invention 9 e  9 44 17 19 0.86 0.87 0.93 0.88  0.022 402 Steel of Invention 10 f 10 29 18 20 0.87 0.88 0.91 0.88  0.013 402 Steel of Invention 11 g 11 35 17 13 0.88 0.91 0.90 0.90 −0.020 417 Steel of Invention 12 h 12 29 16 12 0.88 0.89 0.89 0.88 −0.002 409 Steel of Invention 13 i 13 29 14 17 0.90 0.91 0.88 0.90 −0.017 405 Steel of Invention 14 j 14 25 14 22 0.88 0.89 0.88 0.88 −0.011 396 Steel of Invention 15 k 15 42 12 22 0.90 0.91 0.89 0.90 −0.012 403 Steel of Invention 16 l 16 39 11 17 0.88 0.89 0.88 0.88 −0.009 412 Steel of Invention 17 m 17 27 16 20 0.87 0.88 0.87 0.88 −0.010 400 Steel of Invention 18 n 18 38 17 18 0.91 0.92 0.90 0.91 −0.017 398 Steel of Invention 19 o 19 27 15 18 0.87 0.88 0.92 0.89  0.018 400 Steel of Invention 20 p 20 39 10 22 0.87 0.88 0.90 0.88  0.007 402 Steel of Invention 21 q 21 36 12 43 0.90 0.91 0.90 0.91 −0.011 246 Comp. steel 22 r 22 26 17 21 0.82 0.83 0.82 0.83 −0.010 458 Comp. steel 23 s 23 32 13 22 0.87 0.88 0.87 0.88 −0.010 292 Comp. steel 24 t 24 37 16 21 0.81 0.85 0.81 0.83 −0.040 410 Comp. steel 25 u 25 28 12 19 0.82 0.87 0.81 0.84 −0.058 405 Comp. steel 26 v 26 33 19 18 0.83 0.90 0.82 0.86 −0.077 405 Comp. steel 27 w 27 30 18 24 0.82 0.87 0.82 0.85 −0.054 402 Comp. steel (underlined = outside the scope of the present invention, *= outside the recommended range)

As shown in Table 3, all of Steel Nos. 1, 2 and 6 to 20 are Steels of the Invention produced by using a steel species satisfying the requirements specified for the component composition of the present invention under the recommended hot rolling conditions, as a result, allowed to satisfy the requirements specified for the microstructure of the present invention, and in the steels, both the indicator of drawability and the surface hardness after a carburizing heat treatment meet the acceptance criteria. It could be confirmed that a hot-rolled steel sheet exhibiting a predetermined hardness (strength) after a carburizing heat treatment while ensuring good drawability can be obtained.

On the other hand, Steel Nos. 3 to 5 and 21 to 27 are Comparative Steels failing in satisfying at least one of the requirements specified for the component composition and the microstructure in the present invention, and in the steels, at least one of the indicator of drawability and the surface hardness after a carburizing heat treatment does not meet the acceptance criteria.

For example, Steel No. 3 satisfies the requirements for the component composition, but since all of the heating temperature before hot rolling, the rough rolling temperature, the finish rolling temperature, the rapid cooling stop temperature, and the coiling temperature are outside the recommended range and too low, grains having <001> direction and <110> direction as the crystal direction in the rolling direction are excessively formed, resulting in poor drawability, among others, the anisotropy of r value.

Steel No. 4 satisfies the requirements for the component composition, but since the sheet thickness after hot rolling is outside the specified range and too large, a grain having a sheet-plane orientation of (123) plane is lacking and the grain grows, resulting in poor drawability.

Steel No. 5 satisfies the requirements for the component composition, but since the rolling reduction in the final pass of finish rolling is outside the recommended range and too small, a grain having a sheet-plane orientation of (123) plane is lacking and grains having <001> direction and <110> direction as the crystal direction in the rolling direction are excessively formed, resulting in poor drawability, among others, the anisotropy of r value.

In Steel No. 21 (steel species q), the hot rolling conditions are in the recommended range but the C content is too low, resulting in poor surface hardness after a carburizing heat treatment.

In Steel No. 22 (steel species r), the hot rolling conditions are in the recommended range but the C content is too high, resulting in poor drawability.

In Steel No. 23 (steel species s), the hot rolling conditions are in the recommended range but the Mn content is too low, resulting in poor surface hardness after a carburizing heat treatment.

In Steel No. 24 (steel species t), the hot rolling conditions are in the recommended range but the Mn content is too high, resulting in poor drawability.

In Steel No. 25 (steel species u), the hot rolling conditions are in the recommended range but the Al content is too low, resulting in poor drawability.

On the other hand, in Steel No. 26 (steel species v), the hot rolling conditions are in the recommended range but the Al content is too high, resulting in poor drawability as well.

In Steel No. 27 (steel species w), the hot rolling conditions are in the recommended range but the N content is too high, resulting in poor drawability.

From the above, the applicability of the present invention could be confirmed.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention.

This application is based on Japanese Patent Application (Patent Application No. 2013-219468) filed on Oct. 22, 2013, the contents of which are incorporated herein by way of reference.

INDUSTRIAL APPLICABILITY

The hot-rolled steel sheet of the present invention exhibits good cold formability during processing, and, after a carburizing heat treatment, displays a hardness on a predetermined surface as well as at a predetermined deep portion from the surface and is excellent in the wear resistance and anti-fatigue properties, and therefore, is useful for, for example, clutches, dampers, gears, etc. used in automobiles. 

1. A hot-rolled steel sheet excellent in drawability and surface hardness after a carburizing heat treatment, wherein: the hot-rolled steel sheet has a sheet thickness of from 2 to 10 mm; and the hot-rolled steel sheet comprises in mass % (hereinafter, the same applies to chemical components) C: from 0.05 to 0.30%, Mn: from 0.3 to 3.0%, Al: from 0.015 to 0.1%, N: from 0.003 to 0.30%, iron, and unavoidable impurities; a microstructure of the hot-rolled steel sheet mainly comprises ferrite and pearlite; and, with respect to all grains including the ferrite and the pearlite, existing at a position of t/4 in depth (t: sheet thickness), an area ratio of grains having a sheet-plane orientation within 10° from (123) plane is 20% or more, a total area ratio of grains having a crystal direction within 10° from <001> direction and grains having a crystal direction within 10° from <110> direction, in a rolling direction, is 25% or less, and an average grain size of the all grains is from 3 to 50 μm.
 2. The hot-rolled steel sheet according to claim 1, wherein, in the unavoidable impurities Si is 0.5% or less, P is 0.030% or less, and S is 0.035% or less.
 3. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet further comprises at least one member of the following (a) to (f): (a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%); (b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%); (c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%); (d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%); (e) Sb: 0.02% or less (exclusive of 0%); and (f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).
 4. The hot-rolled steel sheet according to claim 2, wherein the hot-rolled steel sheet further comprises at least one member of the following (a) to (f): (a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%); (b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%); (c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%); (d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%); (e) Sb: 0.02% or less (exclusive of 0%); and (f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%). 